1. Field of the Invention
This invention relates to control of input signals for analog-to-digital converters.
2. Description of the Related Art
Many analog-to-digital converters (ADCs) require that each analog input potential (voltage) not exceed the supply voltage. Many applications, including energy meters require use of additional devices and/or level-shifting circuits to allow the ADC to conform to this requirement. In the past, several approaches have been proposed to handle these problems.
A first approach provides an on-chip mid-supply voltage and creates a virtual ground relative to this supply at the analog input terminals. This approach allows use of a resistor for connection to the analog signal source. The corresponding input signal current is transformed into a voltage signal and is processed on the chip. Some difficulties encountered with this approach are (1) the current to voltage transformation must be at least as linear as is required for ADC operations, (2) each ADC must reject any noise associated with the mid-supply voltage source, and (3) these circuits require additional chip real estate and require additional heat dissipation for the additional current.
A second approach generates an on-chip voltage supply using a charge pump to turn switches on and off. Some variations of this approach are disclosed in U.S. Pat. No. 5,872,469, issued to Nestler. One difficulty encountered here is that a rail-to-rail input signal common mode is not available. Further, the required pump voltage is rather large and may damage an IC fabricated using a deep-submicron technology.
What is needed is an approach that provides a rail-to-rail input signal common mode and allows use of voltages below the ground voltage (zero) established by a standard voltage supply. Preferably, this approach should be flexible enough to allow provision of any reasonable below-zero voltage and should not require excessive additional chip real estate or heat dissipation.